Electrical measuring instrument



NOV.l2 ,1946." s msg 2,411,010

. ELECTRICAL MEASURING INSTRUMENT Filed June 9, 1944 s sheets-sheet 1 2s l9 fl i v J5: i 3.;

| i l I P l l v l' l l l I 1 L J 1 1 v Fig.4. 77 I T s6 7: 71 45 Inventor.

Allen 6. stims'oh,

14 I I by His Attorneg.

W- A. G. STIMSCN 2,411,010

ELECTRICAL umsunms INSTRUMENT Filed June 9, 1944 s Sheets-Sheet 2 Inventor: Allen 6. Stimson,

by mttorneg.

Nov. 12, 1946. A. G. s'nMsoN 2,411,010 '1 ELECTRICALMEASURING INSTRUMENT Filed June 9, 1944 3 Sheets-Sheet 3 J I I i Allen 6. Stimson,

. Iriv entor:

Hus Attorneg.

. Patented Nov. 12, 1946 General Electric New York Company, a corporation of Application June 9, 1944, Serial No. 539,546

2 My invention relates to, electrical measuring instruments and particularly to miniature instruments for measuring frequency, although certain features of the invention areapplicable to measuring instruments generally. An important object of my invention is to provide a compact, rugged, lightweight measuring instrument of goodaccuracy which is easy to assemble. The instruments to be described were designed for use on aircraft where it is especially desirable that instruments be small insize and light in weight. The features of my invention which are believed to be novel and patentable will be pointed out in theclaim appended hereto. Certain structural features of the instrument described herein are claimed in a divisional application Serial No. 588,196, filed April 13, 1945.

For a better understanding .of my invention, reference is made in theiollowing description to the accompanying drawings in which Fig. 1 represents an improved circuit arrangement for a frequency meter embodying my invention; Fig. 2 is a vector diagram explanatory of the theory of operation of the frequency meter of my invention; Fig. ,3 is a diagrammatic representation of a frequency meter embodying my invention; Fig. 4 represents connections for a wattmeter em- 1 Claim. (01. 172-245) in series and, except for the split arrangement to balance the coil assembly with respect to the armature shaft, might be considered as a single coil, since they are connected in boosting relation. The armature circuit includes a condenser l, a

. variable resistance 8, and a variable reactance 9.

The armature circuit thus comprised is connected in parallel with the field coil I0 across the source I I, the frequency of which is to be measured. For the purpose of representing a, practicable example, it will be assumed that the normal frequency of the source of supply is 400 cycles and'that the scale range of the frequency meter is from 350 to'45 0 cycles.

The field coil I 0 is of high reactance and its current represented by vector 12, Fig. 2, lags approximately 90 degrees behind the applied voltage represented byvector l3. The armature circuit is tuned and the phase angle of its current varies with the frequency. At app'roximatelyrated frequency of 400 cycles the armature circuit is I preferably tuned for resonance so that its current bodying'certain features of my invention; Fig. 5

represents a plan view and Fig. 6 a side view of a measuring instrument embodying my invention, the latter viewshowing a portion of a casing for the instrument; Fig. 7 is an exploded view of the coil, laminated field core, and supporting plate used in my invention; Fig. 7a shows one half of the field core assembled; Fig. 8 represents an exploded view of the armature assembly and supporting structure used in the frequency meter of my invention; Fig.9shows the preferred shape of field pole tips and armature core for use in a, wattmeter; Fig. 10 represents the type of armature used in a wattmete'r; Fig. 11 shows the armature circuit of the frequency meter energized from a secondary coil on the filled core to provide temperature compensation; and Fig, 12 is a vector explanation thereof.

First, I will explain the theory of operation of my frequency meter in connection with Figs. 1, 2, and 3. The instrument has a stationary represented by vector I 4 is in phase with the voltage l3 at this time. Since I B is 90 degrees out of phase with the field current I2, no torque due to current flow will be present at this time. The armature, however, is provided with a tiny magnetic vane l5 positioned with its magnetic axis at right angles to the axis of the armature coils, which vane is attracted by the flux across the armature air gap and positions the armature so that its coil axis is at right angles to the field fiux axis across the gap under these conditions,

U-shapedfield core i energized by a voltage coil and has a moving armature winding split into two coaxial coils 2 and 3 mounted on either side of the armature shaft t and a pointer 5 cooperating with a scale 5. The connections for measuring frequency are as represented in Fig. 1 where it is seen that the armature coils are connected and at this time the pointer is positioned to'read near the center of the scale at the 400-cycle graduation. Now when the frequency goes below 400 cycles, the armature current will leadthe voltage 83 as represented by vector M, and a down-scale instrument torque represented by" vector 56' will exist, producing a down-scale deflection. At frequencies above 400 cycles the armature current will lag the voltage l 3 as repre- .sented by vector l4", and an up-scale torque represented by vector I6". will move the pointer up-scale from center. The magnitude of this up-scale and down-scale torque may be increased for a given armature current by increasing the values of capacitance and reactance in the armature circuit in comparison to the resistance, and

the scale distribution changed accordingly. When the details of the armature assembly are described, it will be pointed out thatthe torque of the magnetic vane 15, which opposes the up-scale and down-scale instrument torques de- I scribed, is alsoadjustable. The center restoring a and light in weight.

of 11,500 ohms.

. to reduce the space requirements fiat surfaces (see rigs. 5

torque furnished by vane I 5 varies with the voltage and makes the frequency meter substantially independent of voltage variations.

'It is desirable that the armature current be kept at a low value and that the physical dimensions oi the circuit elements I, 8, and 9 be small After the initial calibration adjustments these circuit elements have fixed values. Without intending to limit my invention but to give a practicable set of values for the different circuit elements and assuring a 110-volt supply, I may use a coil at l having 1800 turns of 0.008 inch copper wire, .425 henry, and 56 ohms resistance. The two armature coils may have 600 turns each of copper wire, the inductance 9 may have 'henrys, and the capacitance at "I may be a .03 microfarad condenser. The resistance of the armature circuit is of the order Under these conditions the field current will be of the'order of 120 milliamperes and the armature current of the order of 10 milliamperes at rated voltage and frequency and with the aid of features to be described hereinafter, the entire instrument and circuit devices may be housed in a cylindrical casing having outside dimensions 2% inches in diameter and 3 inches in length. I desire to point out at this point that the instrument above described may be used as a position indicator by maintaining the voltage and frequency constant and varying the tuning of the armature circuit as by varying the reactance I, for example.

Field structure The laminated magnetic field core i is designed for minimum weight for a given iron loss, to facilitate assembly in a prewound coil and of the assembled instrument. The core with its coil i0 and coil supporting plate I1 is shown disassembled go permits of an accep in Fig. 7. The core comprises six stacks of la'm- I I. to 22 inclusive. The stacks 2| and 22 be stamped with the same inations are similar and may die, and include pole one end with the yoke or coil enclosed parts which are the longerleg portions thereof and which are assembled side by side. The stack sections piece sections integral at yoke part of the core,

ll, 2|,, and 22 are assembled as one group orunit as shown in Fig. la, and comprises one pole piece and one half the yoke, part 22 being between parts II and 22, and these parts riveted together by rivets that pass through the aligned rivet holes 24 and 25. the. same rivet passing through the holes which are numbered alike inrlgfil.

Core parts ll, 2i, group with part 2| eted together to form one half of the yoke. two half yoke portions coll I! from opposite ends, part 2| above and flat against part 22, until the facing surfaces at 20, Fig. I, abut and the facing surfaces at 21 abut. and the bolt openings numbered 2! are aligned andthose numbered 2| are aligned. Bolts 22' 22', Fig. 5, are then passed through the openings respectively, and the corresponding numbered openings in the plate II. The assembled eore structure with its supp rting plate i1 is then clamped together and against the upper and 6) of pedestals 3| insulating support and wall ll adapted to. fit into a cylindrical casing 32 as best shown in Fig. 6. The pole piece parts of the assembled core iaminations are also clamped and 22 are assembled as a the other pole piece and The non-polar ends of the rising from a circular that the laminations assembly obtained.

between I! and 23 and rivare then slid through the w same. Hence,

(5 magnitude but without 4 to the supporting plate II by screws 33 and-ll which also clamp the support I! for the armature assembly to the upper pole piece surface of the field core as shown in Figs. 5 and 6. The openings for the screws 33 and 24 are numbered 33' and 34' respectively. The supporting plate ll has peripheral recesses at 26, 21, 28, and II positioned to receive the large heads of the rivets 40 (see Fig. '1) which are used-in the holes 2|, 25, etc., to rivet the laminations together. This facilitates quick assembly and aids in a rigid eonstructlon. The splitcore arrangement facllitates- It will be evident that the generally U-shaped field core described above has its laminations parallel with the plane of the U and is appreciably thicker and of larger cross section in the pole piece portions than in the yoke portions by reason of the pole piece parts I8 and is, Fig. '1, which lie entirely above the plane of the remainder or the core in a direction at right angles to the plane of the laminations. The thickness of the pole piece portions or the core is increased with respect to the yoke portion of the core by the depth of these laminated pole piece stacks l8 and it. One advantage or this is that I can operate the yoke portion of the core at a higher:

fiux density than would be advisable for the pole piece portions and thus save material in the yoke section, and correspondingly reduce the inner and outer diameters of the coil II and the volume and weight of copper used therein for a. given number or ampere turns and coreloss. At 400 cycles for which the frequency meter is intended, the core loss is likely to be considerable unless certain precautions are taken. The construction 'ble core loss without sacrificing coil space. The laminations 2| and 22 which thread the coil III are preferably made of the order of only 0.005 inch in thickness, which reduces the core loss in the higher flux density while the laminatlons 01' parts I8, It, 20, 22 which operate at appreciably lower flux density and where there is greater need ofstructural rigidity, are preferably made of the order of 0.014 inch in thickness. It is to be noted of the different core parts are not interleaved, as this would not permit of different thickness laminations in parts 20 and 2|, forexample, which lie in the same plane, nor would it permit of the ease of assembly and dis- Instead of interleaving the laminations, certain core parts as a whole overlap and abut against other core parts at the junction of pole pieces and yoke sections and are then clamped rigi y o ether.

To reduce further the core loss of the laminations, the rivets and screws used therethrough are kept as near the periphery of the core and out of the main fiux path as is possible. As shown at 28 and 29 in laminated parts 2i and and as 22, 1-espectively. the bolt openings are not closed by magnetic material. -where flux can pass outside of a bolt or rivet, it will be noted that the cross section of the magnetic material op the outside of all rivets and bolts is substantially the it any flux does go outside the bolts and rivets, that part of the flux will evidently stay outside all rivets and bolts around the entire periphery of the core. which would tend to induce currents therein oi the same phase and a return circuit and as at right angles to the plane of the laminations towards the scale plate 6. This has the advantage that the scale plate may be placed lower down than would be the case if the coil were centered in the vertical direction with the pole pieces, as viewed in Fig. 6, and requires less offset of the pointer and saves a corresponding amount of space in the over-all height of the instrument as picturedin Fig. 6.

The armature assembly and support All parts of the armature, its pivots, lead-in spirals, magnetic damper, armature stop, zero adjustment, and scale plate are supported by the support 35 (see Figs. 5, 6, and 8), and all of these parts are removable from the instrument as a unit by taking out the two screws 33 and 34. The support 35 is an integral die casting of nonmagnetic, corrosion-resisting material such as aluminum. The scale plate 6 is fastened thereto by two screws 4! (see Fig. 5). The support 35 has a lower forward horizontally extending part 52 which supports the lower bearing 43 and lower armature lead-inspiral supporting and adjusting element 55. The upper bearing 85 and I by the flux is inserted an aluminum damping vane 58 secured on the armature shaft 59.

The armature outer peripheral portion, with the shaftinserted tions as follows:

upper lead-in spiral adjusting means 46 are supported by a strap 6! which is fastened to the lateral and forward extending top part of the support 35 by screws 58 entering into threaded openings 59. The two spiral adjusting means 85 and are clamped in place coaxially with the axis of rotation of the armature under the come pression of resilient slightly dish-shaped friction washers 50 which permits of their being rotatively adjusted readily but without danger of accidental movement from adjusted position by vibration.

Beneath the top bearing bridge strap 57 there is a central recess in the support 35 into which there loosely fit a kidney-shaped permanent magnet 5i and spaced return flux keeper 52 comprising the stationary part of a magnetic damper for the armature. The damping magnet parts 5| and 52' are secured near the upper end of a strap 53 which fits into an elongated vertical slot 55 in the rear side of support 35 as viewed in Fig. 8, and secured in place by a screw which enters through a hole '55. The damping magnet parts may thus be polarized as a unit and added last during assembly of the instrument parts and kept clean'of magnetic particles. The strap 53 in the frequency meter also supports a removable armature stop 56. The stop 55 has a screw part threaded in the support 53 and a forward'ceram'ic' bushing part of insulating material which extends freely through the opening 57 in the support 35 and into the path of swing of the armature coils 2 and 3 to the rear of the armature as viewed in Fig. 8, and serves to stop their swing at suitable limits in both directions. The permanent magnet 5| is polarized as indicated in Fig. 8 so as to produce a flux across the air gap between it and the flux return part 52, and into this gap so as to be cut through the bottom of the cup and the cup opening upward. This cup shape provides added strength and rigidity to the damping vane and provides space andprotection within the cup for the upper spiral lead-in wire 60 and the collar to which the pointer 5 and the balancing arms 6i are secured. The balancing arms Bl are at right angles to each other and 45 degrees from the pointer forward of and in the same plane with the damping cup member 58 where they are accessible for adjustment of their weights. This arrangement allows of balancing with two counterweights instead of three as is usual. It also allows of quick balancing with few opera- With the instrument shaft held horizontal and with one balancing arm vertical, the armature is balanced with the other or horizontal balancing arm weight. Then the armature is rotated degrees so that the other balancing arm is horizontal and its counterweight is shifted until a balance is obtained.

Theinstrument shaft 59 is a hollow bronze tube with the upper and lower pivots 62 and 53 pressed into its upper and lower ends. The magnetic vane I5 is secured to the shaft within the coils and is adjustable about its .center support stud which goes through the shaft 59 so that the effective length of the vane and its torque in gives approximately the correct restoring torque for the frequency meter described. When correctly adjusted it is permanently secured in place. g y

The armature coils 2 and 3 are shell-less and are held to the shaft by two flat-surfaced bushings 64 of insulating material fitting against the inner surfaces of the coils and to which the coils are secured at top and bottom by cement andlashings. The armature coils are wound with formex wire with the turns cemented together into a solid mass and are sufiiciently stiff to provide their own support without using a shell or other coil form support. This gives an exceptionally high ratio of useful armature torque to armature weight and space. On the shaft below the coils theremay be a washer 65 of mica to protect and insulate the lower leadin spiral 56. The lead-in spirals of the frev, quency meter are adjusted to have minimum assembled, the total height of the assembly is approximately 1% inches. This assembly together with the scale plate which is omitted in Fig. 8 may be removed asa unit from the pole piece assembly by removing the two dowel screws 33 and 3.5, Fig. 5, and when this assembly is in place in the field and the dowel screws are tightened, the armature coils are correctly positioned in the field air gap.

The circuit elements of the frequency meter comprising the condenser l, the resistance 8,

and the reactance 9 are housed'in the same casing 32 (see Fig. 6) with the instrument and are preferably mounted on the rear wall of. the insulating partition base 31 which supports the instrument as indicated. The supporting enclosure for the condenser 1 may be held between the partition SI and a cross plate indicated at'i'l by bolts 68. The casing 32 may comprise two telescoping cylindrical parts 89 and 10 both secured to the base partition SI of the instrument. Casing part Iii which is the outer rear telescoping part is held in place by nuts on bolts 68. Casing part 69 is held in place by screws, one of which is shown at H, and which are accessible when the casing part I0 is removed. 12 represents the electrical terminals of the instrument. Frequency meters and wattmeters such as described are now being built for use on airplanes, enclosed in cylindrical casings less than'three-inches in total length and about 2% inches in diameter.

When used as a wattmeter the connections are as represented in Fig. 4. The wattmeter armature coil is represented at 13, and the armature circuit is the potential circuit and contains an adjustable resistance H for calibration and a fixed inductance 15 to obtain correct phase angle adjustment between the field and armature. fluxes. The field 16 is preferably energized from a current transformer 11 connected in the current circuit of the line 18 metered.

In the wattmeter I prefer to use an iron core 19 in the armature and change the shape of the pole pieces to that represented in Fig. 9. The same split core and offset pole piece assembly of the laminations are employed as in the frequency meter, the only difference being in the pole face shape of the pole piece laminations. In the frequency meter the pole tips were square to obtain better magnetic vane, center restoring torque. A conventional style of wattmeter armature is used as represented in Fig. 10, but the cup-shaped damping vane 58 and armature balancing arrangement 6| previously described is used thereon. The lead-in spirals 60 and 88 are here used in place of an iron vane for zero restoring torque. The same armature assembly support 35 and damping magnet SI, 52 shown in Fig. 8 are used. The opening 51 in support 35 which in the frequency meter contained a removable armature stop is used in the wattmeter to hold a screw which supports a ceramic armature stop 80 (Fig. 9) and a holding screw ll entering from the rear of such opening is threaded into support 35 to securely hold the part 53 in correct position. The core is held in place between parts one of which is shown in part at 82 (Fig. 9) which extend laterally from the support 35 just inside the armature coil and grasp the core I! from the top and bottom.

In case the frequency meter is likely to be subjected to considerable temperature variations, a further improvement may be had by employing the circuit shown in Fig. 11 where the armature circuit, including coils 2 and 3, is energized by and the excitation voltage of said tuned armaa secondary winding" inductively coupled with the field coil ill instead of being connected across the line II as in Fig. 1. In case the coil l0, Figs. 1, 3, and 11, increases considerably in temperature so that its resistance component increases in comparison to its reactance, the field flux is is likely to shift as represented by the dotted vector 12', Fig. 12, at any given frequency. If the armature circuit is connected across the line as in Fig. 1, there is no corresponding shift in its phase position due to temperature rise. If, however, the armature circuit, including coils 2 and 3, is energized inductively by transformer action from field coil in acting as a primary of a transformer. any shift in the'phase position of the flux in the core i due to rise in temperature of the winding It will produce a corresponding shift in the armature voltage because here the armature voltage is referred to the field flux I: rather than to the linevoltage ii. Thus, at a given frequency, if the field flux shifts from It to II, Fig. 12, the armature voltage phase position will shift from H to ll, which tends to compensate for the temperature error that would otherwise exist due to this shift in field flux and armature current-is substantially unaffected by the changes in field resistance with temperature changes. The armature circuit is nevertheless tuned and shifts as in Fig. 2 with frequency variations. The change in power factor of the secondary armature circuit in Fig. 11 due to changes in frequency has an insisniflcant effect upon the power factor of the primary field coil circuit ll, because the secondary transformer burden represents a very small percentage of the input to coil l0.

In accordance with the provisions of the patent statutes I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

An alternating current electrical measuring instrument comprising a magnetic field core structure and energizing coil therefor, a moving coil armature within the field produced by the field core structure, said armature having a tuned circuit and the instrument operation being due to changes in the phase angle of the fluxes produced by the field and armature by reason of a shift in the phase angle of the armature flux in response to a variable to be measured. and a secondary coil on the field core from which said armature circuit is energized by transformer action from the field coil acting order to maintain a substantially fixed phase relation between the field flux of the instrument ture circuit.

' ALLEN G. S'I'IMSON.

the field flux phase position, since in Fig. 11 the phase relation between the as a primaryin. 

